Miniature electric motors commonly use oil impregnated sintered bushings for the bearings because they are very economical. The bearings primary function is to support a rotor shaft. They can also function as a thrust bearing by confronting an abutment or spacer on the rotor. The abutment is often provided by a fiber washer or similar low friction material supported axially by a step in the form of a spacer or collar fixed to the shaft or a change in diameter of the shaft, thereby limiting axial movement of the rotor. In some applications, the washer is not used, with the spacer directly contacting the end face of the bearing. The bearing and the abutment are designed to make surface contact with each other.
However, when the bearing and the abutment are misaligned so that the faces are not parallel, we find that the edge of the abutment contacts the bearing face and gouges a hole or groove in the face of the bearing. This causes significant friction between the two parts, reducing the useful power output of the motor. As these motors are very small, the power loss may be significant and has been known to prevent further operation of the motor. The misalignment may need be only 1 or 2 degrees to cause problems. The misalignment may be due to improper fitting of the bearing or the abutment or due to assembly and manufacturing tolerances. These causes may lead to misalignments of 5 degrees or more, if not carefully controlled.